The present invention relates to a head positioning system for a magnetic recording/playback apparatus and, particularly, to a head positioning system using a stepping motor and intended to achieve enhanced positioning accuracy.
In the conventional magnetic recording/playback unit mounting detachable magnetic disk such as a floppy disk, the magnetic head is required to have a positioning accuracy within a certain tolerance in order to retain the compatibility with other units. The tolerance, which varies depending on the number of tracks in unit length (track density) and the configuration and dimensions of the magnetic head, has a value substantially equal to the tunnel erasure gap width when a head of the tunnel erasure type having guard bands on both sides of data is used. Accordingly, as the track density increases, higher head positioning accuracy is required.
FIG. 1 is a perspective view showing briefly as an example the conventional head positioning mechanism employed widely in floppy disk drive units. The arrangement shown in the figure includes a stepping motor 1, a pulley 2 press-fitted on the drive shaft of the stepping motor, a steel belt 3, a rail 4 affixed on the chassis (not shown) of the floppy disk drive unit, a carriage 5, a head arm 6, an upper magnetic head 7, a lower magnetic head 8, a chuck 9 for mounting a magnetic recording medium, i.e., floppy disk, (not shown), and a reference track sensor 10.
The head positioning mechanism arranged as mentioned above operates as follows. First, the stepping motor 1 affixed on the chassis rotates incrementally in response to the control signal so that the pulley 2 is rotated incrementally. The rotation of the pulley 2 causes the carriage 5 to move linearly along the rail 4 by being driven by the steel belt 3. Namely, the stepping motor 1 rotates in increments of a certain angle, and accordingly the carriage 5 moves linearly in increments of a certain distance. Accordingly, the upper and lower magnetic heads 7 and 8 mounted on the head arm 6 which is affixed on the carriage 5 are moved linearly in increments of the unit distance. By setting the unit distance equal to the track pitch (the reciprocal of the track density), the upper and lower magnetic heads 7 and 8 are positioned to any track position.
The reference track position sensor 10 detects the edge of the carriage 5 to indicate that the heads 7 and 8 have reached the reference track position (generally, the position of the outermost track).
The positioning error of the magnetic recording/playback apparatus using such a head positioning system as described above is caused by: (1) indexing error of the stepping motor (generally, around .+-.3% of a step), (2) error of the pulley diameter, (3) error of attachment to the chassis, (4) chucking error of the magnetic disk, (5) expansion or contraction of the magnetic disk due to a thermal and humid environment, and (6) misalignment of the upper and lower heads. The total amount of these errors is appreciably large in the currently prevailing magnetic recording/playback apparatus, and the restriction on the aforementioned tunnel erasure width hampers the realization of a much increased track density.
Next, the indexing error of the stepping motor will be described in more detail. FIG. 2 is a graph showing the stepping motor output torque plotted against the rotational angle, and FIG. 3 is a chart used to explain the indexing error of the stepping motor. In both figures, the curve and line indicated by M represent the output torque and error when the motor rotates clockwise, while the curve and line indicated by N represent those of counterclockwise rotation.
The stepping motor stops at a point of zero output torque, but once the motor is loaded it behaves with hysteretic output torque characteristics due to the frictional torque. On this account, the motor stops at different positions depending on the rotational direction as shown in FIG. 2, resulting in an angular error as shown in FIG. 3 that varies at each track.
Next, the angular error of the stepping motor caused by the switching of drive voltages during the step movement and quiescent state of the motor will be described in detail. Generally, a stepping motor used in a floppy disk drive unit is energized by a higher voltage (12 volts in general) as shown in FIG. 4 during a track traversing movement in order to achieve the accurate and fast positioning of the head. Then, upon expiration of a certain interval (t.sub.1) when head positioning has been completed, the motor is kept energized by a lower voltage (5 volts in general).
FIG. 5 is a chart used to explain the angular error of the stepping motor. In the figure, the dashed line indicated by M represents the angular error during the step movement, while the solid line indicated by N represents the angular error after the step movement has completed. Different angular errors result for both moving states due to unbalanced currents flowing in the excitation windings at the moment of switching the drive voltage, and yet these angular errors vary at each track.